1. Field of the Invention
The invention concerns a method and device for improving the pose accuracy of a mechanism having tolerances in a workspace, the mechanism being movable in at least one axis and having an effector, wherein at least one effector object is fixedly mounted on the effector and is eccentric relative to the at least one axis of the mechanism and at least one reference object is arranged in the workspace so that it is fixed relative to the mechanism. A computer system with measurement and control programs is operatively connected to the mechanism. The effector and reference object form a signal trigger/signal detector pair suitable for enabling the triggering and detection of at least binary signals, wherein the totality of the signal poses of the detector relative to the trigger device in which a signal is triggered on the detector may be described by at least one non-trivial characteristic equation.
2. Description of the Related Art
A number of methods for improving pose accuracy of a mechanism are known in the art. For example, FR-A-2 696 969 discloses a calibration method in which a laser beam is fastened at the last segment of a robot and a measurement plane close to the robot is used as reference. The robot approaches a series of hand poses (which are not more closely specified) in which the laser beam meets the measurement plane. After this, the coordinates of the contact points on the measurement plane are identified photographically. From these and from the associated joint configurations from the estimated values for the pose of the measurement plane and from the pose of the laser relative to the hand, the robot parameters are calculated by means of a variation of the Newton calculus of observations method (Levenberg-Marquart method).
In the primary embodiment of this reference, the measurement plane is a mirror or opaque projection screen and the laser beam which meets it is photographed by a camera positioned in front of the mirror. The use of markers on the projection screen is not explained any more closely and is apparently intended to serve the creation of a relation between the camera and projection screen. The measurement plane may also be designed as an optical matrix sensor.
In accordance with further embodiments, two measurement sequences are photographed. Between the two measurement sequences either the laser is mounted at another point, the position of which with respect to a tool support, e.g. point of the hand, is known or the position of the mirror is alteredxe2x80x94the orientation is not mentioned in this reference.
The altered poses are not specified any more closely. The following explanation is given: if the measurement sequence is selected unintelligently, several values may come into question mathematically for identification of the parameters. To prevent such a problem, two different mirror poses are suggested. Problematical is the fact that, using a laser beam, it is not possible to identify the position of the laser or the tool relative to the root of the hand completely. In addition, two intersecting laser beams are used to identify the 5th and 6th parameters of the tool position. Moreover, the system is inexact due to the distortion of the image of the measurement plane by the camera.
Further attempts to identify robot parameters are apparently contained in an essay by Newman, Osborn, xe2x80x9cA new method for kinematic parameter calibration via laser line trackingxe2x80x9d, Proc. Int. Conf. Robotics and Automation, USA, Atlanta (1993), p. 160-165. In this reference, a laser beam is fixedly set up in space close to a robot. A special detector is fastened to the hand of the robot that consists of a planar, rectangular light sensor divided into four separate quadrants. The quadrants meet at a center-point. Each of the four quadrant sensors provides a brightness value. The robot moves the hand successively in various measurement poses defined by the fact that the brightness values provided by the four sensors are identical. The robot parameters are calculated using Newton""s calculus of observations from the joint configuration of the measurement poses and the estimated values for the pose of the laser and the detector relative to the hand. The authors describe a principal structure of the experiment and then report on results of a two-dimensional simulation of their principle on the basis of a two-dimensional robot with two-joints. Not all kinematic parameters may be determined by this method. In particular, the pose of the robot relative to a prescribed coordinate system cannot be identified but only dimensionless parameters, thus making it impossible to derive an absolute size standard of the robot. It is not explained what has to happen in three-dimensional space if the sensor plane does not stand perpendicularly on the laser beam.
The re-calibration of small parameter modifications is described by an essay xe2x80x9cAutonomous robot calibration using a trigger probexe2x80x9d by Zhong et al. in the US magazine Robotics+Autonomous Systems, 18 (1996) S. 395-410. Three plates are fastened in the vicinity of the robot which stand exactly perpendicularly with respect to each other. The robot takes up an omni-directional mechanical probe and approaches a series of hand poses which are not specified more closely and in which the probe touches the plate or triggers the internal contact of the probe. The associated joint configurations are evaluated by a neural network which supplies the robot parameters as result. True sizes cannot by determined by this method as only relative modifications are recognized. Some information is lacking, for example, the true distance between robot and plates, the authors ascertain.
The disclosures of WO96/30171 and WO 93/11915 describe methods and devices for the calibration of the axes of motion of industrial robots.
In WO 96/30171, a calibration device is used which consists of a calibration beam, e.g. a laser in the workspace of a robot, and an associated interrupter detector. A sphere with known radius is mounted on the hand of the robot. The robot heads for a series of hand poses, not specified more closely, in which the calibration beam of the sphere is interrupted. The calibration parameters are calculated by means of the Newtonian-Gauss method from the associated joint configurations, the estimated values for the robot pose, and the pose of the laser relative to the hand. In the preferred embodiment, the calibration beam has to stand perpendicularly on the x-y plane defined by the robot basis. With certain exceptions, six calibration parameters are calculated for each axis.
The accuracy of the calibration parameters may be increased by calculating them several times. In doing so the calibration beam is put into various positions in the workspace. The calibration parameters are then calculated as the mean value of the calibration parameters for the various beam poses. To obtain the greatest variations between the robot configurations used, the robot may be equipped with several calibration beams the pose of which is selected in such a manner that the greatest possible differences between the robot configurations during the various measurements are achieved.
According to the WO 93/11915, a calibration body is used which consists of a cuboid with exactly parallel lateral sides in the workspace of the robot. A sphere with known radius is mounted on the hand of the robot. The robot heads for a series of pairs of hand poses, not specified more closely, in which the sphere touches the cuboid once on any arbitrary side of the cuboid and then again on the opposite side. The presentation and the manner of the subsequent calculation indicate that the second contact point has to lie exactly on the perpendicular of the cuboid point opposite to the first point.
The calculation of the robot parameter takes place in iteration steps. In each step, the relevant coordinate differences of the associated pairs of hand poses are determined first of all on the basis of the current approximation values for all parameters sought by the inventor. If the sum of the squares of these differences deviates from a desired value resulting from the known cuboid dimensions, a Gauss-Newtonian step is carried out.
In one embodiment, the calibration body has to be aligned to the coordinate axes of the robot base. In a further embodiment, the orientation of the calibration body can be arbitrary. Apparently, three additional equations are necessary to identify the three additional orientation parameters.
Neither systems supply exact values as obviously the mathematical and kinematic concepts have not been recognised.
It is an object of the present invention to provide improved methods and devices for the identification of all parameters influencing the pose or the pose accuracy of a mechanism in its true size for as exact as possible direct control of target poses. It is a further object of the present invention to provide an improved method and device for determining pose accuracy that has a favourable cost/utility ratio and does not use traditional measuring instruments, obstructive auxiliary objects in the workspace or manual teach-in methods.
Furthermore, the method and device according to the present invention will determine the exact pose of a reference point on the mechanism with respect to its environment and with respect to an object in the workspace or between objects in their true size.
The object of the present invention is met by a method for improving the pose accuracy of a mechanism in a workspace, wherein the mechanism is movable in at least one axis with tolerances and includes an effector, at least one effector object is mounted via a rigid connection to the effector eccentric to the at least one axis of the mechanism in an estimated pose with a tolerance in position and orientation, at least one reference object is arranged in the workspace with a tolerance in position and orientation, and a computer system is connected to the mechanism having a measurement control program, a parameter identification program, and a mechanism control program, the at least one effector object and the at least one reference object forming at least one trigger/detector pair comprising a signal trigger device and a signal detector for triggering and detecting a binary signal, wherein a totality of signal poses of the signal detector relative to the trigger device in which a signal is triggered on the signal detector is described by at least one non-trivial characteristic equation, said method comprising the following steps:
(a) selecting a proximity sequence N of a finite number of proximity poses for the at least one trigger-detector pair, each of the proximity poses being located in the vicinity of a respective one of signal poses, wherein the proximity sequence N is selected such that the following criteria are fulfilled:
DG(N)xe2x89xa7DG(AI)/15, whereby the distance on an arbitrary straight line G between two neighbouring points of the projection of the proximity sequence N onto G is at the most DG(N)/4, wherein
DG is a function which maps each subset of the set AI to the distance between those two points of the projection of this subset on G which are at maximum distance from each other on G;
AI is the space of all those reachable effector poses of the given mechanism which result from elementary kinematic calculations on the basis of the known mechanism model which in turn is afflicted with tolerances;
G is an arbitrary straight line which contains at least two points of SI; and
SI is a subset of AI which is denotes the space of proximity poses and is defined by the totality of all those effector object poses where a signal would be expected according to a mathematical calculation based on the parameter values of the known tolerance-afflicted mechanism model, the estimated pose of the reference objects in space, and the estimated pose of the effector object on the mechanism.
(b) searching for a nearby signal pose for each of the proximity poses consecutively through movement of one of the at least one effector object and the mechanism using a simple search algorithm until a signal pose is detected;
(c) passing a momentary joint configuration of the mechanism onto the computer system after the detection thereof in said step (b) and storing the momentary joint configuration in the computer system as a data record;
(d) using a parameter identification program to identify the true values of one of the parameters influencing the pose accuracy of the mechanism and user-specific subsets of this parameter set, whereby a scaling factor is used for the identification of all length-parameters.
The scaling factor may be identified by moving an effector object into two pose sets A and B of cardinality, whereby information about the distance between the poses of A and those of B is known. Alternatively, the scaling factor may be identified using at least three calibration objects comprising at least one effector object and at least two reference objects, wherein the at least one effector object is moved into signal poses with respect to the two reference objects with a known relative pose to each other. Furthermore, the three calibration objects may alternatively comprise at least two effector objects and at least one reference object, wherein the at least two effector objects with known relative pose to each other are moved into signal poses with respect to the at least one reference object.
The distance between the poses of the sets A and B or between the locations, calibration objects, or effector objects respectively amountsxe2x80x94for the purpose of error dampingxe2x80x94to more than ⅙ of the diameter of the workspace xcex94, preferably xc2xexcex94 and the diameter of the workspace is defined by the maximum of DG(AI) where G ranges over all straight lines G and a mean value calculation is carried out for the determination distances.
The method according to the present invention may be used for complete or partial re-calibration of the mechanisms or of subsections of the mechanisms.
The following pairs of reference/effector objects, which are in general denoted calibration objects, are examples which may be used and which may optionally be interchanged:
i) signal detector and straight electromagnetic wave/cylindrical interrupter rod;
ii) signal detector and electrically conductive wire/contact rod;
iii) laser beam/light-sensitive matrix area;
iv) electrically conductive plane, conductive contact rod;
v) guidance with contact threshold on rotary axis/guided rod on rotary axis;
vi) point-shaped or planar signal detectors/plane of electromagnetic waves; and
vii) wedge-formed electromagnetic wave, several signal detectors.
The object of the present invention is met by a device for improving the pose accuracy of a mechanism and for pose measurement of objects in the work space, including a computer system comprising measurement control program, a parameter identification program, and a mechanism control program and a mechanism moveable in at least one axis which has an effector, the mechanism being connected to the computer system. At least one pair of devices comprising calibration objects includes an effector object rigidly connected with the effector and mounted eccentrically to the at least one axis of the mechanism and a reference object fixedly arranged relative to the mechanism in the workspace, each pair comprising a signal trigger device and a signal detector for binary signals. The device also includes an installation for determining a scaling factor.
This device is used in the application of the method for pose measurement of mechanisms and objects in the workspace, absolutely or relatively to each other.
The object of the present invention is also met by a method and a device for improving the pose accuracy of mechanisms in a workspace including a mechanism moveable in a least one axis and afflicted with tolerances which has an effector at its disposal, at least one effector object is mounted via a rigid connection to the effector ecentric to the at least one axis of the mechanism in an estimated pose with tolerance in position and orientation, at least one immaterial reference object in the workspace which is arranged fixedly relative to the mechanism in an estimated pose having a tolerance in position and orientation, and a computer system with a measurement control program and a mechanism control program, wherein the at least one effector object and the at least one reference object form a trigger/detector pair suitable for enabling the triggering and detection of at least binary signals, the totality of the signal poses of the detector relative to the trigger device in which a signal is triggered on the detector is described by at least one non-trivial characteristic equation, the method including the following process steps:
(a) selecting a proximity sequence N such that it contains at least one proximity pose of the at least one effector object arranged at a reference object neighbouring a target pose;
(b) successively searching for one nearby signal pose through the motion of one of the effector object and mechanism via a simple search algorithm for the at least one proximity pose via detection;
(c) passing a momentary joint configuration of the mechanism onto the computer system and storing the momentary joint configuration as a data record;
(d) using the computer system to calculate, for each data record, the incorrect pose of the mechanism in the workspace resulting mathematically on the basis of the mechanism parameters currently known to the controller; and
(e) calculating a correction movement from the difference between the signal poses and the associated incorrect poses through elementary interpolation procedures and elementary error compensation algorithms, the correction movement compensating the deviation of the pose actually steered for by the mechanism control from the target pose, whereby a scalar factor, which was determined from the exactly known pose of the reference object, is used for the calculation of the correction movement between the incorrect poses and the signal poses.
Summarising, the present invention identifies efficiently and precisely the parameters influencing the pose accuracy of a generic mechanism at a very favourable cost/utility ratio.
Pertaining to the present invention, sensors (joint encoders) existent in the mechanism are used as well as simplest additional measuring devices which in accordance with the basic function principle may comprise the simplest binary sensors or detectors.
Apart from this, the task of identifying only a certain subset of parameters in a re-calibration efficiently (e.g. such for which it is known that they are altered during operation of the mechanism more quickly than others due to wear and tear) is solved. This means that appreciable expenditure can be saved over the prior art.
In industrial practice, the identification of the pose of a mechanism with respect to the spatial position of a work cell is particularly important.
The method of the present invention may further be supported by a learning system which for its part recognises the residue errors of the calibration process and, through training at the actual mechanism, compensates these errors in its control of the mechanism. The learning system may be integrated fully into the overall process thereby increasing significantly the efficiency of the learning system over previous isolated solution approaches.
A method similar to calibration, denoted here as interpolation, is used for alternative or additional local improvement of the pose accuracy whereby no additional devices are necessary.
Besides the calibration of a mechanism, a specific variation of the method pertaining to the present invention solves the task of identifying precisely the pose of the effector of a (not necessarily calibrated) mechanism relative to the reference objects or a reference coordinate system, workpieces relative to the effector of (not necessarily calibrated) mechanisms, or between arbitrary objects and/or mechanisms with limited, one-time preparation expense and extremely low apparatus expense.
Through specialisation of the method according to the present invention, devices and methods may be gained which are suitable specially for calibration (restriction to approx. 2 to 4 reference objects) or specially for pose measurement (design of special, one-axis measurement mechanisms and special reference objects with the aid of which (indirectly) the pose of arbitrary objects to another may be identified).
The superiority of this shape-adjusting method over previous calibration methods is shown in practical application at least by the following advantages:
simple conceivable installation of the measurement devices;
no calibration of the measurement devices necessary;
lowest costs and service-friendliness through extremely simple measurement setup;
measurement without contact possible so that there is no wear and tear on the measurement set-up;
execution of calibrations at minimum preparation expense and, due to this, frequent (e.g. daily) repetition possible;
guarantee of absolute accuracy of mechanisms over their complete life-term;
the measurement set-up can be integrated completely in the mechanism thus eliminating additional installations in the workspace of the mechanism; and
calibration of subsections of mechanisms possible, through this appreciable
savings in time.
Through this, there are appreciable cost and personnel savings for the user of mechanisms which require calibration while a high degree of accuracy of these mechanisms and resulting high manipulation quality is guaranteed. This results, for example, in a high production quality when used on industrial robots.
The method pertaining to the invention is now to be explained in detail. In doing so, the individual devices and installation or method steps and terms will be explained in connection with their function as this is more comprehensible than a summarising abstract explanation.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.